tests/compose: Add a test for --write-commitid-to

This is used in critical paths like pungi, so let's be sure it works;
the semantics are a bit subtle as it overrides setting the ref.

Closes: #1161
Approved by: jlebon

tests/compose-tests/test-write-commitid.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -xeuo pipefail
+
+dn=$(cd $(dirname $0) && pwd)
+. ${dn}/libcomposetest.sh
+
+prepare_compose_test "write-commitid"
+runcompose --write-commitid-to $(pwd)/commitid.txt
+wc -c < commitid.txt > wc.txt
+assert_file_has_content_literal wc.txt 64
+echo "ok compose"
+
+# --write-commitid-to should not set the ref
+if ostree --repo=${repobuild} rev-parse ${treeref}; then
+    fatal "Found ${treeref} ?"
+fi
+echo "ok ref not written"